mri test 1

principal components of MRI hardware

-magnet

-operating console

-system electronics

gantry made of

superconducting magnets

cryogens made of

liquid helium

temp is _ to keep magnet cool

near absolute 0

Diameter of superconducting magnet

40 cm

protocols to position part at

isocenter

__ that are independent

3 sets of secondary coils

RF coil is nearest to

patient

gradient coil closest to 

aperture

shim coils between

gradient and primary magnet

uniform field intensity

shim coil

Produce pulsed gradient magnetic fields (switched on and off rapidly)

gradient coils

Oscillates at frequency of hydrogen nucleus, produces perpendicular field

radiofrequency coils

T/F Other than the table itself, there are no moving parts in the gantry

true

Controls for image acquisition and image processing
 Emergency shut off***

operating console

generates fundamental resonance frequency used by the system to excite the magnetic nuclear spins

frequency synthesizer

increases signal output from the frequency synthesizer so adequate RF
power is applied to RF coils

RF amplifier

”ramping up” when first installed

magnet power supply

power supply

gradient coil

takes parameters set by technologist and applies them, sends to correct system

Sequencing system – pulse programmer

2 types of digital system in MR machine

signal acquisition

image processing

Types of Magnets Used to Generate B₀

permanent

electromagnets (resistive, superconducting)

Criteria for selection of MRI system

field strength

siting limitations

increasing field strength increases 

size of fringe magnetic field

component of B₀, extend outside of magnet and its housing

fringe magnetic field

B₀ field intensity for permanent magnet

.01-0.3 T

permanent magnet has

low power requirements

no cooling requirements

no magnetic fringe field

field intensity for resistive electromagnet

up to 0.3 T

magnetic homogeneity of resistive electromagnet

10-50 ppm

resistive electromagnet has a __ system

water, heat exchanger cooling

power of resistive electromagnet

80 kW

fringe field of resistive electromagnet

2 m distance to 0.5 mT

Indefinite flow of electrical current and magnetization

superconductivity

field strength of superconducting electromagnet

0.5 to > 5 T

magnetic homogeneity of superconducting electromagnet

0.1-5 ppm

superconducting electromagnet has what feature?

cryogenic cooling

power of superconducting electromagnet

20 kW

fringe field of superconducting electromagnet

10 m distance to 0.5 mT

The container that houses the superconducting wire and cryogens (liquid helium)

cryostat

superconducting advantage: increased MR signal produces __ allows shorter exam times

higher SNR

increased SNR means

higher spatial and contrast resolution

superconducting magnet can be __ more than other magnets

homogenized or shimmed

limited site selection in superconducting magnets because of

fringe magnetic field (can be reduced with shielding)

Once superconducting magnet is brought up to field

it does not require external electrical power to maintain B₀

coil windings must be kept at

cryogenic temperature – helium periodically replenished

occurs when a superconducting magnet warms and the electromagnetic coil becomes resistive

quench

quench may occur from

lack of helium or mechanical trauma

consequences of quench

-can destroy magnet

-gases will replace oxygen in room

controlled quench

ramps down magnet

secondary magnets

shim coils

gradient coils

RF coils

shim magnetic field uniformity

homogeneity

homogeneity specified in

parts per million

__ makes field more homogenous

shimming

Shim coils create small magnetic fields that help make the field more

homogenous and that can be adjusted automatically

gradient coils localize MRI signals and create

gradient field in B₀

time required to energize or switch off a gradient coil

maximum gradient amplitude

slew rate

1 to 40 mT/m

maximum gradient amplitude limits

minimum time to echo

switch off gradient coil produces

thump thump thump

higher gradient produces

pure tone

what causes noises heard during MRI?

Lorentz force

3 sets of gradient coils

X, Y and Z

Produces RF pulse and puts energy into patient

transmit probe

Receives signal from patient

receive probe

Intensity of emitted RF signal and sensitivity of the signal received from the patient are

maximum in a volume approximately equal to the diameter of the coil

Receive RF probes affect the signal-to-noise ratio (SNR)

quadrature coils

body coils

head/extremity coils

surface coils

phased array/matrix coils

Improve SNR by decreasing signal from multiple directions

quadrature coils

quadrature coils more homogenous for

RF transmission and reception

surface coils only

receive signal (inhomogeneous)

coils that can image any anatomy

body

energy, wavelength, and phase

electromagnetic spectrum

magnetic resonance images made with radiofrequency

10-300 MHz

MRI originally called

nuclear magnetic resonance 

pt is stimulated so ___ is emitted from the body, the signal is then used to produce the image

electromagnetic radiation

considered the father of MRI

Felix Bloch

described __ with Bloch equations

nuclear magnetism

Bloch equations explain that because a nucleus spins on an axis, it has a

magnetic field

field is called 

magnetic moment

why MRI

no ionizing radiation

high contrast resolution

MRI spatial resolution depends on

tissue characteristics

MRI can obtain direct transverse, sagittal, coronal, and oblique plane images

multiplanar imaging

place cursor on lesion within a region of interest and retrieve NMR spectrum

spectroscopy

similar to CT, controls for post-processing, mechanical incrementation
for patient localization, move patient to isocenter

operating console

Graph of signal intensities created to identify the tissue and location within the field

fourier transformation

at isocenter, gradient is

0

deals with stationary electric charges

electrostatics

proton has one unit of

positive charge (stationary)

electron has one unit of

negative charge (free to move)

1 couolomb =

6.24 × 10¹⁸ electronic charges

When electrons are added to or removed from a material

electrified

Excess electrons move to neutral or e- deficient object

contact electrification

two objects touching causes transfer

friction electrification

Mass or energy transferred without actual contact between objects

induction electrification

a force field called the __ is associated with each charge

electric field (E)

lines of force begin on __ charges and end on __ charges

positive

negative

The magnitude of the electric field is the force on a unit charge

E= F/Q

E is

electric field intensity

F is

force on charge

Q is

electric charge

like charges __, unlike charges __

repel

attract